The zoo keepers is worried because the number of visitors  __       smaller and smaller.

In the past decades, sea ice _____in the Arctic as a result of global warming.

The information that    will become your ability once you use it．

For some time the expert __________ people of the health risks caused by passive smoking. He     says he won’t stop until all the smokers around him decide to quit smoking. 

 We    on it for several hours but we haven't yet reached any conclusion.